Double rotation Y-cut crystal units such as SC-cut crystal units are excellent in thermal-shock characteristics and the like. Therefore, the double rotation Y-cut crystal units are used for highly-stabilized crystal oscillators of a constant-temperature type, for example. In the SC-cut crystal unit, there is not only a C mode that is for the main vibration, but also a B mode leading to an abnormal oscillation (frequency jump) due to a vibrational frequency approaching that in C mode. Therefore, it has been demanded to suppress the B mode so as to secure an oscillation in the C mode.
FIGS. 3a and 3b are diagrams for explanation of one example of a related-art crystal unit, in which FIG. 3a is a cutting azimuthal chart of an SC-cut crystal element, and FIG. 3b is a view of a crystal element.
The crystal unit is composed of an SC-cut crystal element 1. A principal surface of the crystal element 1 is perpendicular to the Y″ axis of newly rotated crystal axes (X′, Y″, Z′) that are rotated to the left that is a counterclockwise rotation at θ° (approximately 33°) and φ° (approximately 22°) centering on the X axis and the Z axis of crystal axes (X, Y, Z). In short, the crystal unit is composed of a double rotation Y-cut crystal plate, in which the principal surface (Y surface) perpendicular to the Y axis is rotated to the left at θ° and φ° centering on the X axis and the Y axis.
In reality, for example, the double rotation Y-cut plate is rotated at θ° centering on the X axis, and is thereafter rotated at φ° centering on the Z′ axis that is newly generated. Alternatively, the double rotation Y-cut plate is rotated to the left at φ° centering on the Z axis, and is thereafter rotated to the left at φ° centering on the X′ axis newly generated. Generally, θ is referred to as a direction angle, and φ is referred to as a tilt angle, and a direction angle θ has an effect on a temperature characteristic (peak temperature) and a tilt angle φ has an effect on the CI.
Further, as shown in FIG. 4, the crystal is a trigonal crystal. Therefore, X axes (solid lines) serving as electric axes exist at intervals of 120°, and Y axes (chain lines) serving as mechanical axes exist so as to be perpendicular to the X axis. Incidentally, FIG. 4 is a cross sectional view, in which the direction passing through the sheet plane is the Z axis serving as an optical axis, and which is perpendicular to the Z axis. Accordingly, the tilt angle φ(22°) as an SC-cut crystal element is (30−φ)° by use of the Y axis rotated by 30° in the counterclockwise direction as a standard, that is, an angle rotated in the clockwise direction by 8° by use of the rotation angle 30° from the X axis as a standard.
The crystal element 1 is formed into a rectangular geometry longer in the X′-axis direction, for example, and the X′ axis is defined as a length L, the Z′ axis is defined as a width W, and the Y″ axis is defined as a thickness T. Excitation electrodes (not shown) are formed on both principal surfaces of the crystal element 1, and leading electrodes extend from both sides of one end thereof, for example. Then, both sides of the one end of the crystal element 1, from which the leading electrodes extend, are held by unillustrated means, and the crystal element 1 is hermetically encapsulated to form the crystal unit.